1099-R Box 2a is blank - what is taxable?

Its ok if your Form 1099-R box 2a is blank. It's quite common for that box to be left blank.

You should enter your Form 1099-R exactly as you have received it. TurboTax will determine how much of your distribution is taxable based on your 1099-R entries and your answers to the follow-up questions in the interview process.